The Structure and Variability of Accreting Supermassive Black Holes

Abstract

Accretion onto a supermassive black hole (SMBH) powers some of the most luminous objects in the Universe. Yet the structure of this flow is not well understood. Standard accretion disc models match only to zeroth order in predicting substantial energy dissipated in optically thick material. Closer inspection shows challenges in matching the observed spectral shapes, as well as a complete failure in predicting the observed variability.

In this thesis I attempt to address some of these issues. I start by examining the spectral energy distribution (SED), to constrain the energy generating structure. Using a combined sample from eROSITA and Subaru's Hyper SuPrime Cam, I show a systematic evolution in the energy generating structure as a function of mass-accretion rate. At moderate to high mass-accretion rates the flow is predominantly formed of an optically thick, dense, disc structure. However, below a (mass-scaled) mass-accretion rate of the disc collapses, leaving behind an optically thin, geometrically thick, X-ray hot plasma.

While the SEDs show a wide range in SMBH accretion structure, they do not give significant detail on the physical nature of the flow. For this I turn to variability, now focusing on disc dominated systems, using the local active galactic nucleus (AGN) Fairall 9 as a guide. X-ray reverberation was the previously accepted solution to optical/UV variability, since the accretion disc cannot vary on observable time-scales. Here I show that this is not the case. The X-rays do not carry sufficient power to drive the observed optical/UV variability signature. Instead, it is likely that the disc is intrinsically variable itself, departing strongly from standard theory. I develop a physical model for this, and show that it produces variability signatures that qualitatively match the observations. However, I then perform a further, more detailed, study using the newest data, which shows significantly increased complexity beyond what my model predicts. Specifically, these data show non-stationary changes in the inner structure of the flow on remarkably short time-scales. Understanding how this could occur could provide the key needed to understand accretion in SMBHs.